You didn't say what the ampere hour rating is on the batteries. If they're typical 12v "deep cycle" marine/RV, then I think the number I saw floating around this forum was 62 AH. If that's the case, then just replacing those batteries with 6 volt deep cycle golf cart batteries such as Trojan T-105s will get you 225 AH, discharging to 50 percent basically doubles your capacity, so it would be like adding 2 batteries.
Another thing to consider; unless your existing batteries are new, you'll probably need to buy 4 so they would balance out.